Leaf springs or the like are used so as to be stacked in suspension apparatuses of vehicles (trucks or the like). In a production method for this leaf spring, for example, steel is subjected to hot bending forming. FIGS. 1A to 1C are diagrams which show a schematic construction of each processing of bending forming. FIG. 1A is a diagram which shows leading end portion cutting, FIG. 1B is a diagram which shows starting time of winding, and FIG. 1C is a diagram which shows ending time of winding. In the bending forming, first, one end portion side of the steel 1 is mounted on a die 11, and the other end portion side thereof is fixed at a fixing member 12. Next, one end portion of the steel 1 is abutted and pressed by a bending member 13 from one end of a curved portion of the die 11 to the other end thereof. Thus, the one end portion of the steel 1 is bent, and a curved portion 1A is formed.
Next, as shown in FIG. 1A, one end portion of the curved portion 1A of the steel 1 is cut by a cutter 14. Next, as shown in FIG. 1B, while an upper surface of the one end portion side of the steel 1 is pressed by a pressing member 21, the curved portion 1A of the steel 1 is pressed and is wound toward the inside of the curved portion 1A by using a die 22. Thus, as shown in FIG. 1C, an eye portion 3 is formed. The above bending forming is performed on the other end portion of the steel 1, so that the steel 1, which has a leaf portion 2 and eye portions formed at both end portions of the leaf portion 2, is obtained.
Next, the steel 1 is subjected to heating processing (quenching and tempering), so that a leaf spring 10 is obtained. Next, as shown in FIG. 2, shots are injected from an impeller 25 of a shot peening apparatus to a surface (a surface at which the eye portions 3 are formed) of the leaf portion 2 of the leaf spring 10, and the surface of the leaf portion 2 is subjected to shot peening. Thus, compressive residual stress is provided to the surface of the leaf portion 2, and the durability is improved.
Next, since regarding a typical inner surface of the eye portion 3 as shown in FIG. 3, for example, a winding-start portion 4 of the inner surface of the eye portion 3 and a portion 5 facing the winding-start portion 4 have flattened shapes, an inner surface of the eye portion 3 is subjected to reaming. Thus, at least portions 3A are removed, and the inner surface of the eye portion 3 has a true circle shape, so that the leaf spring 10 shown in FIG. 4 is obtained. Next, a bushing (not shown in the Figures) is press-fitted into the inside of the eye portion 3, and the leaf spring 10 is fixed to a vehicle body (not shown in the Figures) via the bushing.
In recent years, the leaf spring 10 has been improved in weight reduction and high stress, so that generated stress at the eye portion 3 is increased. Thus, it has been proposed that shot peening be performed on not only the leaf portion 2 but also the inner surface of the eye portion 3, and compressive residual stress be thereby provided to the inner surface of the eye portion 3. In the shot peening to the inner surface of the eye portion 3, a shot peening apparatus 30 shown in FIG. 5 is used (see Patent Document 1, for example).
The shot peening apparatus 30 is equipped with a nozzle 3 land a reflection member 32. The nozzle 31 injects shots 33. The reflection member 32 has a reflection surface 32A reflecting the shots 33. In shot peening to the inner surface of the eye portion 3, the reflection member 32 is disposed inside the eye portion 3, the shots 33 are injected from the nozzle 31 to the reflection member 32, and the shots 33, which are reflected by the reflection surface 32A of the reflection member 32, are projected to the inner surface of the eye portion 3. In this case, the reflection member 32 is moved in the axial direction of the eye portion 3, so that the shot peening is performed on the inner surface of the eye portion 3, which extends from one opening (shot entrance side opening) to another opening (shot exit side opening) of the inner surface of the eye portion 3.
Patent Document 1 is Japanese Unexamined Patent Application Publication No. H5-138535.